Chapter Four - Section 3

Trail Design Guidelines: Use Modes

While it is most common for use modes to be combined on
trails or within corridors, Iowa Trails 2000 discusses each
mode to ensure that the needs of various users are thoroughly considered.
When combining use modes, the guidelines for each mode should be consulted
and the most stringent should be used (see "Multi-Use
Corridors"). The modes considered include hiking/walking, bicycling,
in-line skating, equestrian, snowmobiling, off-highway vehicles (OHVs), and
motorcycles (canoe trail designation
is covered later). Each of these use modes is described below, and guidelines
are set forth relating to the following design considerations.

Clear Trail Width refers to the width
of the traveled part of the trail that is free of protruding objects and
obstacles, such as trees and overgrown vegetation (see Figure 4-5).

Clear Zones refer to the area on each side
of the trail between the traveled surface and any obstructions, such as
trees, walls, or fences (see Figure 4-5).

Vertical Clearance refers to the height
above the trail which is free from protruding objects and overhead obstructions,
such as tree branches or bridges (see Figure 4-5).

FIGURE 4-5: TRAIL DIMENSIONS

Trail surface refers to the type of surface
on the traveled part of the trail, such as asphalt, concrete, granular, or
alternative. Surface quality is affected by tread obstacles, such as roots
or rocks, and by any openings such as gaps and grates located within the
trail surface.

Drainage refers to techniques used to move
and keep water off the trail and trail embankment.

Alignment refers to the horizontal curvature
of the trail.

Profile refers to the vertical curvature
of the trail.

Edge protection refers to any protective
barrier designed to separate the trail from its surrounding environment,
such as a fence or curb. As a general rule, curbs should not be less than
4 inches in height. Other types of edge protection are discussed, where appropriate,
under each trail mode.

These design guidelines are meant as general recommendations.
Many of the design considerations listed above will be impacted by local
conditions, such as topography, right-of-way width, and intensity of use.
Each trail project is unique, and while these guidelines should be employed
wherever possible, deviations may occur.

Hiking/Walking Trails

Pedestrian facilities can take several forms. Hiking/walking
trails, sidewalks and pedestrian
trails provide different user experiences for pedestrians.

Hiking/walking trails, covered in this section, are facilities
used exclusively by pedestrians, and are typically found in natural areas.
They offer a low-impact means of allowing pedestrians to come in contact
with the natural environment. Hiking/walking trails are used by a variety
of people with a broad range of abilities, skill levels, and desired experiences,
and should be designed to accommodate all persons. New and reconstructed
trails should be made as accessible as possible while maintaining the essential
character of the resource. Furthermore, all trail amenities, such as restrooms,
drinking fountains, and picnic tables should comply with the ADA accessibility
guidelines. Because of their rustic nature, the guidelines for hiking/walking
trails are very general, and trail design will be primarily determined by
site conditions.

Hiking/walking trails should include widened areas at regular intervals
to allow users to pass one another. These widened areas should be at least
5 feet by 5 feet.

In urban or suburban locations, hiking/walking trails should be set back
at least 5 feet from any roadway curb.

Clear Zones

Hiking/walking trails do not typically require clear zones,
since users are moving at relatively slow speeds. In natural areas, underbrush
should be trimmed so that it does not hang over the trail edge or obstruct
the traveled way.

FIGURE 4-6: TRAIL DIMENSIONS FOR HIKING/WALKING
TRAILS

Vertical Clearance

Hiking/walking trails should maintain an 8-foot minimum vertical clearance
(see Figure 4-6). If the hiking/walking trail is used by cross-country
skiers during the winter months, the average snow level should be added
to the 8-foot minimum.

Trail Surface

Hiking/walking trails may be surfaced with wood chips or crushed stone,
or may be made of compacted earth. In any case, the surface should be firm
and stable. It should be noted, however, that wood chips are not considered
an accessible surface.

Any tread obstacles, such as rocks or roots, imbedded into the trail
surface should be less than 2 inches.

Any openings within the trail surface, including on bridges, should not
permit passage of a 0.5-inch diameter sphere and should be perpendicular
to the dominant direction of travel.

Drainage

Because users of a hiking/walking trail will come in direct
contact with the trail surface, drainage is very important. Natural surface
trails can become watercourses during heavy rains, causing severe erosion.
The following methods effectively move water off the trail.

In flat areas, the trail should be cross-sloped or crowned at approximately
2 percent.

Where a trail is benched into a slope, a swale on the uphill side should
be considered to catch water before it crosses the trail.

Culverts may be necessary to move water under the trail.

Disturbed areas should be seeded and mulched or sodded to prevent erosion.

Alignment

Users of hiking/walking trails can navigate even the tightest
of turns. Alignment guidelines are not necessary for hiking/walking trails.

Profile

It is recommended that no more than one-third of the total trail length
for a hiking/walking trail exceed 8.3 percent. In addition, the following
guidelines should be followed:

Trail grade may be 5 percent or less for any distance.

Trail grade may be 8.3 percent for a maximum distance of 200 feet.

Trail grade may be 10 percent for a maximum distance of 30 feet.

Trail grade may be 12.5 percent for a maximum distance of 10 feet.

The trail grade between the maximum grade segments should return to 5 percent
for a minimum distance of 5 feet to allow resting opportunities for people
who have difficulty traveling over sloped surfaces.

If, due to local topography, the trail would be steeper
than the above recommendations permit, switchbacks should be used to lessen
the overall slope.

Edge protection

Edge protection is not required on a hiking/walking trail;
however, if provided it should be at least 4 inches. Pedestrians with vision
impairments tend to adjust their obstacle detection to a slightly higher
level on hiking/walking trails because of all the small obstacles contained
within a natural trail surface. Edge protection that is at least 4 inches
high is much more likely to be detected.

Pedestrian Trails

Pedestrians are typically accommodated with other trail
users such as bicyclists and in-line skaters, within a multi-use corridor.
In some cases, however, pedestrians may be accommodated on an exclusive trail,
as a means of separating pedestrians from faster moving bicyclists and in-line
skaters.

Where pedestrian use is expected, facilities should be
accessible to a variety of people with a broad range of abilities, skill
levels, and desired experiences, and should be designed to accommodate all
persons. New and reconstructed trails should be made as accessible as possible
while maintaining the essential character of the resource. Furthermore, all
trail amenities, such as restrooms, drinking fountains, and picnic tables,
should comply with the ADA accessibility guidelines.

Pedestrian trails, unlike hiking/walking trails, are designed
for a more formalized trail experience. Whereas hiking/walking trails may
be quite rugged, pedestrian trails are typically designed for more leisurely
walking on finished surfaces.

Clear Trail Width

Recommended width for pedestrian trails: 5 feet.

Clear Zones

Because of the relatively slow speed of pedestrians, clear zones are not
necessary.

Vertical Clearance

Pedestrian trails should maintain an 8-foot minimum clearance. If the
hiking/walking trail is used by cross-country skiers during the winter months,
the average snow level should be added to the 8-foot minimum.

Trail Surface

Pedestrian trails, as discussed above, will almost always
exist in conjunction with non-motorized multi-use trails. Their surface,
therefore, should be the same as that used for the adjacent multi-use trail.
Where pedestrian trails occur alone, they may be asphalt, concrete, or granular.
Whenever possible, the surface of a pedestrian trail should be smooth and
free of tread obstacles. Any openings imbedded into the trail surface should
not permit passage of a 0.5-inch diameter sphere and should be perpendicular
to the dominant direction of travel.

Drainage

Pedestrian trails should have a 2 percent cross-slope.

Alignment

Users of pedestrian trails can navigate even the tightest
of turns. Alignment guidelines are not necessary for pedestrian trails.

Profile

It is recommended that no more than one-third of the total
trail length for a pedestrian trail exceed 8.3 percent. In addition, the
following guidelines should be followed:

Trail grade may be 5 percent or less for any distance.

Trail grade may be 8.3 percent for a maximum distance of 200 feet.

Trail grade may be 10 percent for a maximum distance of 30 feet.

Trail grade may be 12.5 percent for a maximum distance of 10 feet.

The trail grade between the maximum grade segments should
return to 5 percent for a minimum distance of 5 feet to allow resting opportunities
for people who have difficulty traveling over sloped surfaces.

Edge protection

Edge protection is not required on a pedestrian trail;
however, if provided it should be at least 4 inches.

Sidewalks

Sidewalks are pedestrian facilities primarily used in cities
and towns. They are typically designed for pedestrians only, and should not
be used by bicyclists. Sidewalks typically offer pedestrian connections within
a community, and are, therefore, an important component of local pedestrian
planning. Guidelines for this type of facility are found in the handbook
"Local Community Planning for Bicyclists and Pedestrians," (Iowa
Trails 2000).

Bicycle Trails

There are extensive guidelines that have been established
for bicycle facilities. Bicycles, however, are unlikely to ever enjoy exclusive
use of a trail facility. In most cases, bicycle trails will also accommodate
pedestrians and in-line skaters on a single paved treadway.

Because bicycles typically travel at higher speeds than
pedestrians, trail geometrics are a major consideration. The AASHTO Guide
is an invaluable resource when designing bicycle trails. The guide gives
detailed information on alignment and profile layout and design.

Recommended width for one-way bicycle trail: 6 feet (Separated one-way
trails in the same corridor should have a minimum 2-foot median between
them).

Clear Zones

Bicycle trails should maintain a minimum 2-foot graded area on each side
of the trail, graded at a maximum slope of 6:1 (see Figure 4-7).

Bicycle trails should maintain a minimum 1-foot buffer zone between the
edge of the graded clear zone and any fixed objects such as signs or trees.
On bridges this guideline does not apply (see Figure 4-7).

The surface of a bicycle trail should be smooth and free
of tread obstacles. In some cases, granular surfacing may be used as an interim
solution. Granular trails can be difficult to maintain, and can be harder
on bicycles than paved trails. In addition, granular surfacing eliminates
use of the trail by in-line skaters. Any decision to use granular surfacing
for bicycle trails should be carefully evaluated.

Drainage

It is very important that bicycle trails are well drained.
Standing water on the trail will adversely affect the trail surface and decrease
the life and quality of the trail.

Bicycle trails should not exceed a uniform cross slope of 2 percent (see
Figure 4-8). Crowning of the trail at 2 to 3 percent is acceptable, but
may be more difficult and costly to construct (see Figure 4-9).

Where a trail is benched into a slope, a swale on the uphill side should
be considered to catch water before it crosses the trail (see Figure 4-10).

Culverts may be necessary to move water under the trail.

Disturbed areas should be seeded and mulched or sodded to prevent erosion.

FIGURE 4-8: TRAIL CROSS SLOPE

FIGURE 4-9: CROWNING OF A TRAIL

FIGURE 4-10: TRAIL WITH DRAINAGE SWALE

Alignment

The design of bicycle trail alignment can be as complex
as roadway design. Many factors must be taken into consideration, including
design speed, the surface type, and sight lines. The AASHTO Guide and "Minnesota
Bicycle Transportation Planning and Design Guidelines" offer detailed
information on alignment and superelevation. In general, a typical curve
radius for a bicycle trail will be approximately 100 feet.

Another issue to consider when designing a trail’s alignment
is visibility on horizontal curves, which is based on stopping sight distance.
Stopping sight distance refers to the amount of time it would take a user
to stop once an obstruction has come into view. As a general rule, the distance
a user can see along the trail should never be less than the distance it
would take that user to stop. Procedures for determining stopping sight distance
are detailed in the AASHTO Guide and should be applied to both alignment
and profile.

Profile

The profile of a bicycle trail is also a major consideration
which requires detailed analysis and design. Issues to consider when designing
a trail’s profile include steepness (or overall grade of the trail) and stopping
sight distance (discussed above). The following recommendations are for general
planning purposes only. Final trail design requires more detailed analysis
based primarily on the AASHTO Guide.

Maximum recommended grade for bicycle trails: 5 percent.

Grades on bicycle trails steeper than 5 percent are possible, but should
be restricted to distances as indicated in the AASHTO Guide.

Stopping sight distance applies to vertical curves (hills)
just as it does to horizontal curves. This consideration is especially important
on downhill sections, as speeds will be higher. As described above, the AASHTO
Guide is an invaluable resource for detailed trail design, and should be
consulted during the final design process.

Edge Protection

Edge protection, typically in the form of fencing, is required
on bicycle trails only in areas where safety is a concern. Such safety considerations
should be evaluated in detail during the final design of the trail. If fencing
is provided, it should be at least 42 inches high. Some possible situations
where fencing might be warranted include:

Locations where the land on either side of the trail drops off steeply.

Locations where sharp curves may cause users to lose control and leave
the trail.

Locations where adjacent uses, such as railroad tracks or active industry,
may cause a threat to trail user safety.

Where fencing is included, rub-rails should be installed
for the safety of bicyclists and wheelchair users. Rub-rails should be installed
at ground level and at the general level of an adult bicyclist’s handlebars.

In-line Skating Trails

In-line skaters are typically accommodated along with other
modes. They will be commonly found along with bicyclists and pedestrians
on multi-use trails. In-line skating trails, therefore, can use the standards
described for bicycle trails (see ">Bicycle
Trails").

On-Road Bicycle Facilities

There is extensive literature relating to guidelines for
on-road bicycle facilities. AASHTO and FHWA, as well as many states, offer
a wide range of guidelines for various types of bicycle accommodations. There
are essentially three types of on-road bicycle facilities: paved shoulders,
shared roadways (including wide curb lanes), and bicycle lanes. All on-road
bicycle facilities should be designed so bicyclists travel in the same direction
as motorists.

Safety is of great concern in the design of on-road bicycle
facilities. Conflicts with pedestrians, automobiles, or other bicyclists
can lead to serious injury. Poorly maintained pavement, snow build-up and
debris can also lead to safety problems. The guidelines listed below are
minimum recommendations only, and site-specific conditions may dictate variations
for safety purposes.

One issue that may impact on-road bicycle facilities is
the presence of rumble strips. Occasionally used on roadways with rural sections,
they will lessen the usable width of an on-road bicycle facility. Rumble
strips "…are not recommended where shoulders are used by bicyclists
unless there is a minimum clear path of 1 foot from the rumble strip to the
traveled way, 4 feet from the rumble strip to the outside edge of paved shoulder,
or 5 feet to adjacent guardrail, curb or other obstacle." (AASHTO Guide,
1999).

On-road bicycle facilities will normally benefit from design
standards required by the roadway itself. Such requirements are sufficient
for the bicycle facility. On-road bicycle facilities should only be designated
on hard-surfaced roadways.

Drainage

The primary drainage issue to consider regarding on-road
bicycle facilities is the existence of roadway drain inlets. Some types of
inlet grates may trap a bicycle wheel or send the rider off course. Bicycle-compatible
inlets are widely available, and these should be used on all roadways where
bicyclists are expected. On rural sections, the cross-slope required by roadway
construction is adequate to drain the bicycle facility.

Mountain Bike Trails

Mountain bike trails are typically rugged, off-road facilities.
They have far less stringent guidelines than non-motorized multi-use trails,
but can accommodate only one type of bicycle. The hallmark of mountain bike
trails is the "single track," which is a narrow pathway with many
hills and sharp turns. Such facilities can vary greatly in difficulty.

Recently, there has been a surge of people who recreate
in off-road wheelchairs that are designed similarly to mountain bikes. However,
not every mountain biking trail will accommodate the additional width of
off road wheelchairs (approximately 28 to 34 inches). Therefore, trail designers
should post objective information about the minimum clear width of the trail,
so people who use off road wheelchairs can make informed recreation decisions.

Clear Trail Width

Desirable width for mountain bike trails: 2 feet (see Figure 4-15).

Clear Zones

Shrubby vegetation should be removed to a distance of 3 feet on each
side of the tread. Established trees and grasses may remain (see Figure
4-15).

Flexible waterbars or swales should be used to remove water from trails.

Special consideration should be given to placement of trails.

Alignment

Alignment of mountain bike trails will primarily depend
on the difficulty of the trail to be constructed. In general, the tighter
the turn, the more challenging a trail may become.

Profile

Maximum overall grade for mountain bike trails: 10 percent. This level
of steepness will allow minor increases or decreases in slope to avoid obstacles.
Dips and inclines should be built into the trail to provide interest and
facilitate drainage.

Edge Protection

Edge protection is not usually required for mountain bike
trails. In areas where safety is of great concern, fences with a minimum
height of 42 inches should be installed.

Equestrian Trails

Trails designed to accommodate horses have a great deal
of flexibility in design. The most important consideration for equestrian
trails is the surface, which should be designed to reduce injuries to animals
and riders. The placement of obstacles is also a key issue for designing
equestrian trails. Some people with mobility impairments are able to travel
by horseback but are not able to walk a horse around obstructions. Therefore,
equestrian trails should not require the rider to dismount to avoid obstacles
while on the trail. In all design elements, the safety of the horse and rider
is paramount.

Tread width refers to the actual traveled surface of the
trail. Cleared trail width refers to the areas where underbrush, branches,
and other obstructions have been removed. In most cases, there will be little
difference between the two, as riders will use the entire cleared area, especially
when passing in opposite directions.

Switchbacks should be used for surmounting slopes greater than the above
parameters.

Edge Protection

Edge protection is not usually required for equestrian
trails. In areas where safety is of great concern, fences should be installed.

Snowmobile Trails

Snowmobile trails are unique among the trail modes considered
in Iowa Trails 2000 because their use will only take place in winter.
This seasonal dependency necessitates some unique design considerations.
In addition, snowmobiles are capable of high speeds, increasing the need
for safety through trail design. As with all motorized trails, signing should
be used to warn non-motorized users of the predominate use mode. In some
situations clearly indicated dual trails can be indicated for the safe sharing
of a corridor by motorized and non-motorized users.

At sharp corners or unusually rugged terrain, the trail should be widened
to accommodate grooming equipment and provide user safety.

The groomed surface refers to the area which is free from
branches, large rocks, brush, stumps, and other obstructions that would create
an uneven and unsafe surface even when the trail is covered with snow.

FIGURE 4-17: TRAIL DIMENSIONS FOR SNOWMOBILE TRAILS

Clear Zones

Snowmobile trails should maintain a 2-foot clear zone on each side of
the groomed surface (see Figure 4-17).

Many snowmobile trails are enjoyed by other trail users
during the summer months. In these situations, the surface should be designed
according to the needs of the additional user. If the trail is not used during
the summer, a variety of surfaces are possible because the trail will be
buried with snow for snowmobile use. The surface should be relatively flat
and free from obstructions as listed above.

Snowmobile trails may exist on an otherwise unprepared surface, provided
that stumps, brush, and other obstructions are removed. Snowmobile trails
within road rights-of-way demonstrate this type of surface.

Snowmobile trails may exist on crushed stone surfacing.

Snowmobile trails may exist on wooden bridges or boardwalks when crossing
watercourses or wetlands.

Placement of snowmobile trails on asphalt surfaces should be avoided,
as studs will cause damage to the asphalt. When implementing a snowmobile
trail along with an asphalt trail, a natural surface corridor should be
provided and clearly marked for snowmobile use.

Alignment

Minimum forward visibility for snowmobile trails: 50 feet.

Minimum radius for snowmobile trail curves: 25 feet.

Where hazards exist (such as a steep drop-off) near a curve, the trail
should be superelevated.

Snowmobile trails should ascend steep slopes at right angles to the contour
lines (directly up the fall line). Ascending such slopes at angles could
cause sliding of snowmobiles and slope erosion.

Edge Protection

Edge protection is not usually required for snowmobile
trails. In areas where safety is of great concern, fences should be installed.

Other Points to Consider

Water crossings: Even though ice may be in place for much of the snowmobiling
season, water crossings without bridges are not acceptable as part of a
snowmobile trail.

Exposure: In order to extend the snowmobiling season, trails should be
placed, wherever possible, to retain snow cover. Tree lines, woods, valleys,
and north-facing slopes are areas that tend to retain snow, and these areas
should be sought out for snowmobile trails.

Signage: The Iowa Department of Natural Resources has developed uniform
signage for snowmobile trails. The DNR’s signage scheme should be used for
all snowmobile trails. These signs should be installed before the first
snowfall and removed in the spring.

Maintenance: Snowmobile trails require a significant amount of maintenance,
since winter storms can take their toll on trailheads, signage, and the
groomed trail itself. Such maintenance issues should be considered during
the initial planning stages of the project.

Noise abatement: There is the potential for disturbance from snowmobile
noise. For this reason, snowmobile trails should be placed as far as possible
from residential areas. Other noise abatement possibilities include placing
the trail behind existing vegetation or within valleys. In addition, sound
monitoring and enforcement should be initiated to ensure that machines do
not exceed the legal limits.

Off-Highway Vehicle Trails (3- and 4-wheeled)

As with snowmobiles, off-highway vehicles (OHVs) are capable
of high speeds, and safety is a primary consideration in the establishment
of design guidelines. OHV trails may exist as either a nodal or linear facility,
with nodal facilities offering looping trails within one designated area
or park, and linear facilities offering connections between riding parks,
communities, and support services. As with all motorized trails, signing
should be used to warn non-motorized users of the predominate use mode. In
some situations clearly indicated dual trails can be indicated for the safe
sharing of a corridor by motorized and non-motorized users.

The Iowa Department of Natural Resources has recently established
a policy regarding the acquisition and development of OHV parks (nodal facilities).
This policy is included in Appendix E.

The following guidelines generally hold true for trails
in both nodal and linear facilities.

OHV trails should be placed on soils that are resistant to erosion. Sandy
soils should be avoided. County soil survey maps should be consulted to
determine the best location for an OHV trail.

The OHV trail surface should be free of logs, large rocks, stumps, brush,
and other obstructions, unless a more challenging experience is desired.
In such a case, some obstacles may be left in place.

Drainage

Improper drainage on OHV trails can lead to rutting and
severe erosion. Trails can be drained by using changes in grade or rolling
drain dips. Waterbars should be used as a last resort, as they increase maintenance
costs.

Variety in grades for OHV trails is recommended, as it increases the
challenge and desirability of the trail, and facilitates drainage.

Minimum slope for OHV trails (for drainage purposes): 2 percent.

Maximum continuous slope for OHV trails: 8 percent.

Maximum grade for shorter slopes (100 feet)on
OHV trails: 15 percent.

Edge Protection

Edge protection is not usually required for OHV trails.
In areas where safety is of great concern, fences should be installed.

Other Points to Consider

OHV parks: Facilities specifically designated for OHV use can offer great
challenge and variety. Such parks are typically designed with a system of
loops, beginning at a trailhead and possibly offering several loops of different
ability levels. OHV parks are likely to be shared by motorcyclists, so loops
should be planned for these users, as well.

Erosion: To reduce the potential of erosion, OHV trails should avoid
unstable soils and provide adequate drainage, especially on steep slopes
and hillsides.

Noise abatement: OHVs may reach noise levels significantly higher than
allowed by the Code of Iowa. Natural buffers such as hills, ridges, and
existing vegetation can help to mitigate noise impacts. To reduce noise
conflicts, OHV parks should have regular sound level monitoring to ensure
all OHVs comply with the Iowa Code.

Motorcycle Trails

Motorcycle trails are very similar to OHV trails in that
they both accommodate motorized recreational vehicles. These two trail modes
often use the same facilities, the only exception being motorcycle-only trails
located in OHV riding areas (see "Other Points to Consider" above).
The following guidelines relate only to variations in trail width, alignment,
and profile associated with motorcycle-only trails. For all other trail elements,
guidelines for OHV trails should be followed. As with all motorized trails,
signing should be used to warn non-motorized users of the predominate use
mode. In some situations clearly indicated dual trails can be indicated for
the safe sharing of a corridor by motorized and non-motorized users.

Clear Trail Width

Recommended width for a one-way motorcycle trail in a wooded area: 3
feet.